Long ago, before Luna became the creature know as Nightmare Moon, and before Sombra and Discord were problems, her and Sombra were friends. Maybe more that that, actually. They were researching ancient alicorn magic, and some of it corrupted Sombra. Yet he and Luna still liked each other, and searched frantically for a cure.

But not every illness can be cured, and not everypony can be saved.

Also, it has sad elements, but there's more funny.